Method and apparatus relating to optical fibers

ABSTRACT

A method of joining a first optical fiber ( 110 ) to a second optical fiber ( 130 ) comprises the steps of: (i) providing a preform element ( 10 ) comprising material defining a primary elongate cavity ( 40 ); (ii) inserting the first optical fiber ( 110 ) into the primary cavity ( 40 ) to form a preform ( 125 ); and (iii) drawing the second optical fiber ( 130 ) from the preform ( 125 ); wherein, the second optical fiber ( 130 ) includes a core region comprising material that has been drawn from the first optical fiber ( 110 ).

This invention relates to the field of optical fibres.

Single-mode and multimode optical fibres are widely used in applicationssuch as telecommunications. Such fibres are typically made entirely fromsolid materials such as glass, and each fibre typically has the samecross-sectional structure along its length. Transparent material in onepart (usually the middle) of the cross-section has a higher refractiveindex than material in the rest of the cross-section and forms anoptical core within which light is guided by total internal reflection.We refer to such a fibre as a conventional fibre or a standard fibre.

Most standard fibres are made from fused silica glass, incorporating acontrolled concentration of dopant, and have a circular outer boundarytypically of diameter 125 microns. Standard fibres can be single-mode ormultimode. They can have more than one core, and they can bepolarisation-maintaining fibres.

Standard fibres are in widespread and routine use and so an establishedtechnology of fibre connectors, splices and input/output couplers existsto insert light into and extract light from them efficiently andcost-effectively. It is therefore advantageous for a new design ofoptical device for telecommunications applications to be made compatiblewith pre-existing standard fibres.

In the past few years a non-standard type of optical fibre has beendemonstrated, called the photonic crystal fibre (PCF) [J. C. Knight etal., Optics Letters v. 21 p. 203]; such fibres have alternatively beencalled holey fibres or microstructure fibres. Typically, a PCF is madefrom a single solid material such as fused silica glass, within which isembedded an array of air holes. The holes run parallel to the fibre axisand extend the full length of the fibre. A region of solid materialbetween holes, larger than neighbouring such regions, can act as awaveguiding fibre core. Light can be guided in this core in a manneranalogous to total-internal-reflection guiding in standard fibres. Oneway to provide such an enlarged solid region in a fibre with anotherwise periodic array of holes is to omit one or more holes from thestructure. However, the array of holes need not be periodic fortotal-internal-reflection guiding to take place; we nevertheless referto such a fibre as a photonic-crystal fibre.

Another mechanism for guiding light in photonic-crystal fibres is basedon photonic bandgap effects rather than total internal reflection. Forexample, light can be confined inside a hollow core (an enlarged airhole) by a suitably-designed array of smaller holes surrounding the core[R. F. Cregan et al., Science v. 285 p. 1537]. True guidance in a hollowcore is not possible at all in conventional fibres.

PCFs can be fabricated by stacking glass elements (rods and tubes) on amacroscopic scale into the required pattern and shape, and holding themin place while fusing them together. This primary preform can then bedrawn into a fibre, using the same type of fibre-drawing tower that isused to draw standard fibre from a standard-fibre preform. The primarypreform can, for example, be formed from fused silica elements with adiameter of about 0.8 mm.

Instead of being drawn directly into fibre, the primary preform caninstead be drawn into a secondary preform with a width that isintermediate between those of the primary preform and the intended finalfibre. The secondary preform can then itself be further drawn down toform the final fibre. Clearly, further intermediate steps can beintroduced if needed. Provision of a secondary preform step can make thefibre fabrication process more reliable and productive. It also permitsthe fabricator to modify the structure at the secondary preform stage.For example, a solid silica jacket can be introduced when the secondarypreform is drawn.

The PCF has a number of technologically significant properties,including (not necessarily simultaneously): endlessly single-modeguidance over a very broad range of wavelengths [T. A. Birks et al.,Optics Letters v. 22 p. 961], a large mode area to carry high opticalpowers [J. C. Knight et al., Electronics Letters v. 34 p. 1347], a widerange of dispersion characteristics [A. Ferrando et al., ElectronicsLetters v. 35 p. 325], high optical nonlinearity [J. K. Ranka et al.,Optics Letters v. 25 p. 25], guidance in multiple cores that may or maynot interact [B. J. Mangan et al., Electronics Letters v. 36 p. 1358],and guidance in air or vacuum in a hollow core [R. F. Cregan et al.,Science v. 285 p. 1537].

Exploitation of the properties of PCFs would be facilitated by simpleand effective means to couple light into and out from these fibres. Inparticular, it would be of great benefit to make PCFs directlycompatible with existing standard fibres. Unfortunately, some of thetechniques of conventional fibre optics are not readily applicable toPCFs. For example, an incautious attempt to fusion-splice a PCF to astandard fibre can cause the air inside the PCF to expand explosively,destroying the joint. If the optical modes of the fibres being joinedare not well-matched in size, even a careful joint will be highly lossy.Furthermore, the problem of pig-tailing more than one input or outputfibre has always proved to be a major impediment to the exploitation ofmultiple-core fibres (both of PCF or standard design).

Tapering techniques involving heat treatment of fibres after they arefabricated can solve some of these problems. For example, heating andstretching of a fibre can be used to yield a low-loss transition thattransforms the size of the guided mode of a PCF [International PatentApplication No. PCT/GB/00599, published as WO 00/49435] but the matchwith a standard fibre will be at best approximate. Alternatively, aspecial PCF can be made that has a high-index core that guides lightconventionally and is otherwise matched to a standard fibre [J. K.Chandalia et al., IEEE Photonics Technology Letters vol. 13 p. 52];there are also large air holes in the fibre cladding so that, when thefibre is drawn down, the result is a PCF-like structure. However, theholes in the standard-fibre end of this structure can cause difficultiesduring fusion-splicing, and there is no benefit at all when trying tointerface with multiple-core PCFs. Hence a method of pig-tailing a PCFwith standard fibres, such that each core of the PCF has a low-lossoptical transition to its own standard fibre, would be of great utility.

An object of the invention is to provide a method of joining fibres thatovercomes the disadvantages of prior art methods and an opticalwaveguide comprising joined fibres.

According to the invention there is provided a method of joining a firstoptical fibre to a second optical fibre, the method comprising the stepsof: (i) providing a preform element comprising material defining aprimary elongate cavity; (ii) inserting the first optical fibre into theprimary cavity to form a preform; and (iii) drawing the second opticalfibre from the preform; wherein, the second optical fibre includes acore region comprising material that has been drawn from the firstoptical fibre.

The primary elongate cavity may communicate with a face of the preform.The primary elongate cavity may communicate with opposite faces of thepreform. Preferably, the material of which the preform element iscomprised is a glass, such as fused silica. The preform element may bemade from substantially a single material. Preferably, the material ofwhich the preform element is comprised is the same as a claddingmaterial of the first optical fibre.

The material of which the preform element is comprised may be undoped.Alternatively, the material of which the preform element is comprisedmay be doped; possible dopants include germanium or a rare-earth dopant.Preferably, at least one dopant species is concentrated in isolatedregions of the cross-section of the preform element (that is, regionsthat are not directly connected to each other). Preferably, each suchregion is no greater in area than four times the cross-sectional area ofthe first fibre. Preferably, the dopants raise the refractive index ofthe material in which they are incorporated. Preferably, a wave-guidingcore is formed in the second fibre from the material having the raisedrefractive index.

Alternatively, at least one dopant species is present throughout thematerial of which the preform element is comprised except in isolatedparts of the cross-section. Preferably, each such part is no greater inarea than four times the area of the first fibre. Preferably, thedopants lower the refractive index of the material in which they areincorporated. Preferably, a wave-guiding core is formed in the secondfibre from the material not having the lowered refractive index.

Preferably, the primary cavity has a substantially circularcross-section. Preferably, the primary cavity has a smallest transversedimension of at least 100 microns. Larger dimensions may beadvantageous; for example possible lower limits include 125 microns, 150microns, 200 microns, 300 microns or 500 microns.

Thus, the cavity may be large enough to accept more than one first fibreat once. Preferably, a plurality of first optical fibres are insertedinto the primary cavity. The plurality of first fibres may then beincorporated into a single core of the second fibre. The second fibremay then perform the functions of a fused fibre coupler since the secondfibre resembles such a coupler with silica supports.

The preform element may have a plurality of primary cavities into eachof which a first optical fibre is inserted. The preform element may thusincorporate more than one hole large enough to accept a fibre, and soprovide an interface between more than one fibre and the cores of amultiple-core second fibre, such as a multi-core PCF. Such an interfaceis particularly difficult to achieve otherwise.

The preform element may include a secondary elongate cavity that doesnot receive an optical fibre. The secondary elongate cavity maycommunicate with a face of the preform. The secondary elongate cavitymay communicate with opposite faces of the preform. Preferably, thesecondary cavity has a cross-sectional area smaller than that of anycavity in which an optical fibre is received. Preferably, the secondarycavity has a substantially circular cross-section. Preferably, thesecondary cavity is at most 500 microns in its largest transversedimension. Smaller dimensions may be advantageous; for example, possibleupper limits include 300 microns, 200 microns, 150 microns, 100 microns,80 microns, 50 microns, 30 microns or 20 microns.

Preferably, there are at least two secondary cavities. More secondarycavities may be advantageous; for example, possible lower limits on thenumber of such cavities include 3, 4, 6, 12 and 18.

Preferably, a hole in the second fibre is formed from the secondaryelongate cavity.

Preferably, the secondary cavities are arranged around the primarycavities in the preform element transverse cross-section. Preferably,the secondary cavities are arranged in a periodic pattern.

Preferably, the preform element, together with one or more first fibresarranged in one or more primary cavities is substantially a large-scalereplica of a functioning photonic crystal fibre. Of course, indetermining whether or not a particular preform is such a replica, onemay need to allow for such changes in the relative sizes and shapes ofthe cavities as can take place during fibre drawing where the cavitiescan be subject to different air pressures. Photonic crystal fibres maybe arranged to have many useful properties; for example, a photoniccrystal fibre may be an endlessly single-mode fibre a multiple corefibre, a dispersion modified fibre, a large mode area fibre, a highlynonlinear fibre, or a hollow-core fibre.

The preform element and the first fibre may thus form aphotonic-crystal-fibre preform. When that preform is drawn down in sizethe resulting reduced structure is a PCF, within which the entireoriginal fibre forms part of a waveguiding core. If the transitionbetween the original fibre and the PCF satisfies the well-knowncriterion for adiabaticity in fibre transitions, light can propagateefficiently between the original fibre core and the core of the PCF. Thecriterion for adiabaticity (described to various approximations in J. D.Love et al., Electronic Letters vol. 22 p. 912 and J. D. Love et al.,IEEE Proceedings J vol. 138 p. 343) defines the maximum rate of changeof a waveguide transition, along the length of the waveguide, thatpreserves low-loss propagation; that maximum can be calculated usingnumerical techniques well-known to those skilled in the art. Thetransition can therefore be a low-loss interface between fibres such asthe original fibre and other fibres such as the PCF.

A photonic crystal fibre may have a cladding comprising an array ofsolid, elongate cylinders embedded in a matrix material, rather than anarray of holes. The preform element may thus comprise a plurality ofsolid, elongate cylinders, which are drawn to form part of a claddingregion of the second fibre.

The first fibre may be a standard fibre. Alternatively, it may be a PCF.The photonic crystal fibre may include at least one core that does notincorporate a first fibre.

Preferably, the preform element is formed by stacking a plurality ofelements. Preferably, at least some of the elements are glass tubesand/or rods. More preferably, all of the elements are glass tubes and/orrods. At least one of the elements may itself be formed from a pluralityof glass tubes and/or rods. Preferably, the elements have asubstantially circular outer cross-section. More preferably the elementshave substantially the same outer diameter. Preferably, the elements arefused together. Preferably, the stack is assembled on a larger scale andthen drawn down in size to form the desired preform element.

Preferably, the primary elongate cavity is formed by omitting at leastone element from the stack. Alternatively, the primary cavity may beformed from the bore of a tubular element in the stack. Preferably, thetubular element from which the primary cavity is formed has a largerbore than that of any tubular element not forming another primarycavity. Preferably, the tubular element from which the primary cavity isformed has a larger outer diameter than that of any tubular element notforming another primary cavity.

The primary cavity may be an interstitial cavity between adjacentelements. The secondary cavity may be formed from the bore of a tubularelement in the stack. Alternatively, the secondary cavity may be aninterstitial hole between adjacent elements.

Alternatively, the preform element may be formed by extrusion.Alternatively, the preform element may be formed by casting of sol-gelmaterial.

Preferably, the preform element is enclosed in an outer jacket.

Preferably, the second fibre is drawn in a fibre drawing tower.Preferably, the drawing of the second fibre proceeds in the same way asthe fabrication of a photonic crystal fibre from a preform. The lowerdrop-off portion of the combination may be retained so as to form adouble-ended device.

Alternatively, the second fibre may be drawn in a fibre-tapering rig.Drawing using a fibre-tapering rig would yield a double-ended structure.Such a double-ended structure may be cleaved to form two single-endedstructures.

Preferably, a hole is prevented from forming in the second fibre bymaintaining differential gas pressures in holes during drawing. Thus, ifthere is excess space in the primary cavity even when the first fibre isinserted, the excess space may be eliminated, as the fibre is drawn, byusing differential pressure to collapse the preform element around thestandard fibre. Similarly, there may be excess space between the preformelement and the outer jacket. Similarly, there may be an interstitialhole, between the stacked elements from which the preform element isformed, that would otherwise form a hole in the second fibre.

Preferably, the size of the hole, in the second fibre, formed from thesecondary cavity is controlled by differential gas pressure. Forexample, collapse of the hole may be prevented by differential gaspressure or the size of the hole relative to the rest of the preform maybe maintained or increased during drawing by differential gas pressure.The differential gas pressure may be provided by partial evacuation ofat least one elongate cavity. The differential gas pressure may beprovided by compression of gas within at least one elongate cavity.

An example of a useful embodiment of the invention is a form of ‘nullcoupler’ (see T. A. Birks et al., Opt. Lett. vol. 19, pp 1964–1966(1994)). A prior-art null coupler is made by fusing two fibres togetherto form a tapered, fused region. However, the fibres fused to form anull coupler are sufficiently dissimilar that substantially no light is,in fact, coupled between them. Rather, each fibre cleanly excites asingle transverse mode of the coupler waist (typically, the fundamentalmode and one or more higher order mode). Null couplers made from morethan two fibres thus enable excitation of more than one higher-ordermode, as each fibre included in the coupler excites a differenttransverse mode (although symmetry considerations prevent excitation ofsome higher-order modes).

Null couplers can be employed in a number of ways, for example aspolarisers (T. A. Birks et al., Opt. Lett. vol. 20, no. 12, pp.1371–1373 (1995)), switches (T. A. Birks et al., Opt. Lett. vol. 21, no.10, pp. 722–724 (1996)) or mode converters. A mode converter is madefrom a null coupler by cleaving the coupler at its waist (where thehigher-order mode or modes are excited). However, cleaving at the waistresults in a thin end-portion of fibre, which is sensitive to externaldisturbances (as light is guided on the fibre's outer surface) andtherefore is not easily handled. Furthermore, the thinness of theend-portion results in it being susceptible to displacements byair-currents, making handling still harder.

A method also according to the invention provides an improved method ofmaking a mode converter similar to the converter based on a nullcoupler. Thus, a core may be provided in the preform, the core having adiameter sufficiently different from the diameter of the core of thefirst optical fibre that substantially no light is coupled between them,the core, the first optical fibre and the preform being arranged suchthat light propagating in the first optical fibre will excite ahigher-order mode in the second optical fibre. In a null coupler, atleast two fibres cores are needed to provide a route into a higher-ordermode of the waist from at least one of them, since one fibre provides aroute into the fundamental mode of the waist. However, it is notnecessary to access one of the cores in mode-converter operation andthus a ‘fibre’ in the form of a dummy core may be built into thepreform. Thus, the core may be provided as a dummy core in the preform.Alternatively, the core may be provided in a third optical fibreinserted into a cavity in the preform. Features of the fibre and corenecessary to achieve excitation of the desired higher-order mode canreadily be determined by a person skilled in the art; for example, anappropriate core size may readily be calculated for a given core size inthe second (i.e., the drawn) fibre.

Preferably, a plurality of fibres are inserted into the primary cavity,the fibres being such that each fibre excites a different higher-ordermode in the drawn fibre.

It may be that one or more fibres are inserted into a further cavity inthe preform element; thus, the fibre or fibres in the primary cavity mayexcite a first set of one or more higher-order modes and the fibre orfibres in the further cavity may excite a second set of one or moredifferent higher-order modes. Of course, there may be still furthercavities containing still further fibres.

Thus, the preform element may provide mechanical stiffness and opticalisolation from the environment for the coupler.

An example of a possible application for a mode converter according tothe invention can readily be appreciated from, for example, A. H. Gnaucket al., Electron. Lett. Vol. 36, No. 23, pp 1946–1947 (2000), whodescribe use of a mode converter to compensate dispersion and dispersionslope of a telecomms optical fibre.

Also according to the invention there is provided an optical devicecomprising a first optical fibre and a second optical fibre having acore region and a cladding region, wherein the core region of the secondoptical fibre comprises material that has been drawn from the firstoptical fibre.

Such a structure differs from the structure of J. K. Chandalia et al.,IEEE Photonics Technology Letters vol. 13 p. 52, in that whereas theformer is a standard fibre at one end, the latter merely resembles oneat its centre but must still have air holes in the cladding.Furthermore, the latter retains grave problems of location and splicingwhen interfacing with PCFs with more than one core.

Preferably, there is a gradual transition from the first fibre to thesecond fibre. Preferably, the transition shape is gradual enough tosatisfy the criterion for adiabaticity, such that the transformationdevice carries light between the first fibre and the second fibre withless than 3 dB of loss. Lower losses are more desirable; for example,possible upper limits for the loss include 1 dB, 0.5 dB, 0.2 dB, or 0.1dB.

Preferably, the device includes a length of preform between the firstoptical fibre and the second optical fibre. Preferably, transitions to apreform having a first fibre exist at both ends of the second fibre.

Preferably, the core region of the second fibre is the first fibre withreduced size. The second fibre may be a standard fibre. Preferably, thesecond fibre includes at least one elongate hole. Preferably, the secondfibre is a photonic-crystal fibre. Preferably, the second fibreincorporates a second core that does not incorporate a first fibre.Preferably, the second core incorporates dopants, such as germanium or arare-earth dopant. The second core may be hollow.

Preferably, the second fibre includes a jacket region; preferably, thejacket region forms an integral part of the second fibre, with no gapbetween it and the rest of the second fibre.

Preferably, the first fibre forms an integral part of the second fibre,with no gap between it and the rest of the second fibre.

The second fibre may have particular properties; for example, it may bean endlessly single-mode fibre, a multiple core fibre, a dispersionmodified fibre, a large mode area fibre a highly nonlinear fibre or ahollow core fibre.

Preferably, the first fibre is a standard fibre. The standard fibre maybe a single-mode fibre. The standard fibre may be made from fusedsilica. The standard fibre may be a telecommunications fibre. Thestandard fibre may have a plurality of cores. The standard fibre may apolarisation-maintaining fibre. The standard fibre may incorporaterare-earth dopants.

Alternatively, the first fibre may be a photonic crystal fibre.

The optical device may comprise a third optical fibre. The core regionof the second optical fibre may include material drawn from the thirdoptical fibre. The second optical fibre may include a second core regioncontaining material drawn from the third optical fibre. Thus, there maybe a plurality of fibres more than one of which leads to the core regionof the second fibre. Each of that plurality of fibres may lead to thecore region of the second fibre. There may be a plurality of fibres morethan one of which leads to a separate respective core in the secondfibre. Each of that plurality of further fibres may lead to a separaterespective core in the second fibre.

Of course, the optical device may have other features corresponding tofeatures described above with reference to the method according to theinvention.

Preferably, the optical device comprises a further core, the furthercore having a diameter sufficiently different from the diameter of thecore of the first optical fibre that the cores couple into differenttransverse modes of the second optical fibre, wherein the first opticalfibre, the further core and the second optical fibre are arranged suchthat light propagating in the first optical fibre excites a higher-ordermode in the second optical fibre.

Preferably, the optical device comprises a further optical fibre,arranged such that light propagating in the further optical fibreexcites a second, different, higher-order mode in the second opticalfibre.

Thus, also according to the invention there is provided a mode convertercomprising an optical device as described above as being according tothe invention. Possible arrangements for the mode converter arediscussed above in relation to aspects of the method according to theinvention.

Also according to the invention, there is provided an optical fibretransformation device, comprising a length of a preform including alength of a first fibre inside a first hole, the other end comprising asecond fibre that is a photonic crystal fibre, and a gradual transitionfrom one structure to the other in between. The length of preform mayhave been partially drawn.

Also according to the invention there is provided a preform elementcomprising a primary elongate cavity large enough to receive a firstoptical fibre and a secondary elongate cavity, such that, when anassembly of the preform element and a first optical fibre is drawn downin size a second optical fibre is formed, the second fibre being aphotonic crystal fibre with a core incorporating the first fibre and acladding incorporating the secondary elongate cavity.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the drawings, of which:

FIG. 1 shows a preform element for use in a method according to theinvention;

FIG. 2 shows the preform element of FIG. 1 partially drawn from a bundleof capillaries;

FIG. 3 shows an optical device according to the invention partiallydrawn from the preform element of FIG. 1;

FIG. 4 shows an end view of a photonic crystal fibre forming part of thedevice of FIG. 3.

FIG. 5 shows a partially drawn preform for forming a mode-coupleraccording to the invention.

Preform element 10 (FIG. 1) is cylindrical and comprises matrix material30, which defines an array of secondary elongate cavities in the form ofelongate, circular holes 20, arranged on a triangular lattice andextending along the longitudinal axis of the cylinder. At the centre ofthe preform element 10, the matrix material 30 defines a primaryelongate cavity in the form of a further elongate hole 40 of diameter150 microns (dimensioned to receive an optical fibre). The preformelement functions as a form of “ferrule”, enabling two optical fibres tobe linked.

The preform element 10 is made by stacking an array of fused silicatubes 70 of diameter 0.8 mm (FIG. 2). Each tube comprises a cylindrical,silica outer portion 90 that defines an elongate hole 80. The stack 60forms a hexagonal pattern with five tubes 70 along each side of thehexagon (only three are shown in FIG. 2 for ease of illustration).However, there is no tube 70 at the centre of the stack 60 but insteadthe innermost tubes 60 define a central hole 100. The stack 60 is drawndown, in a manner well known in the art, in a fibre drawing tower tofuse the tubes together and produce preform element 10 (FIG. 2 showspreform element 10 partially drawn from stack 60). To prevent any of theinner six tubes 60 moving into the central hole 100, spacers can beused, as described in International Patent Application No.PCT/GB00/01249, published as WO 00/60388.

Thus matrix regions 30 in the preform element 10 are formed from outerportions 90 in the stack 60; similarly, holes 20 are formed from holes90 and central hole 40 is formed from central hole 100.

A standard single-mode fibre 110 is stripped of its polymer coating,thoroughly cleaned, and inserted into the central hole 40 of the preformelement 10 (FIG. 3). Additionally, for convenience, a solid silica outertube 120 is positioned around the preform element 10. Preform 125,formed of the fibre 110, the preform element 10 and the jacket 120, isthen drawn into fibre 130 using a drawing tower, in a manner well knownin the art. Standard fibre 110 does not fit tightly in central hole 40and, similarly, preform element 10 does not fit tightly in outer tube120. The pressures in the spaces formed around the fibre 110 and thepreform element 10 are reduced in pressure using a vacuum pump and asuitable gas-tight fitting at the top end of the preform element 10(that is, the end uppermost in the drawing tower, corresponding to theleft-hand end of the device as shown in FIG. 3). Two unwanted gaps arethus not formed in the final fibre 130. Partial evacuation of the spacesalso helps to prevent holes 80 (which are not depressurised) fromcollapsing themselves. Of course, such a method of evacuating a spacebetween an outer tube and an inner element is of general application inPCF manufacture and may be useful in arrangements other than thatdescribed here. It provides a convenient method of removing unwantedgaps and/or preventing the collapse of (wanted) holes during drawing.

The drawing process is stopped before all of the preform 125 is drawnout and the remaining preform 125 is removed from the drawing tower. Asshown in FIG. 3, it has a tapered region 135 that was inside the furnaceand some of the drawn fibre 130 still attached. Such a tapered structureprovides a low-loss transition between the original standard fibre 110,emerging from the end of the preform element 10, and a fibre 130 that isoptically indistinguishable from a PCF made by techniques of the priorart.

An example of an optical device that can be made according to theinvention is a mode coupler 200 (FIG. 5).

Two standard single-mode fibres 260, 270 (of diameter 125 microns) areprovided. A length 280 of fibre 260 has been pre-tapered (that is,narrowed in diameter in a tapering rig beforehand) to a diameter of 90microns. Fibre 270 and part of length 280 of fibre 260 are inserted intoa central hole 250 in a ferrule 210, the hole 250 being big enough topass both fibres. Ferrule 210 also includes an array of holes 220,arranged to provide guidance to light in core 290 of drawn portion 230(only six such holes are shown in FIG. 5, for ease of illustration).

The ferrule 210 and fibres 260, 270 are drawn down on a tapering rig oron a drawing tower to reduce the diameter of ferrule 210 by a factor of30. Core 290 of the drawn fibre thus contains material drawn from fibres260, 270. The drawing is sufficiently gradual that the transition 240between narrowed region 230 and untapered region 210 is adiabatic. Thenarrowed region 230 is then cleaved.

Light entering the unpretapered fibre 270 emerges at the cleaved endfacein the fundamental mode of narrowed section 230, while light enteringthe pretapered fibre 260 emerges in the LP₁₁ (second-order) mode of thenarrowed section 230.

1. An optical fibre transformation device, comprising at one end alength of a preform element including a length of a first fibre inside afirst hole, the other end comprising a second fibre that is a photoniccrystal fibre, and a gradual transition from the first fiber to thesecond fiber.
 2. A preform element comprising a primary elongate cavitylarge enough to receive a first optical fibre and a secondary elongatecavity, such that, when an assembly of the preform element and a firstoptical fibre is drawn down in size a second optical fibre is formed,the second fibre being a photonic crystal fibre with a coreincorporating the first fibre and a cladding incorporating the secondaryelongate cavity.
 3. A method of joining a first optical fibre to asecond optical fibre, the method comprising the steps of: (i) providinga preform element comprising material defining a primary elongatecavity; (ii) inserting the first optical fibre into the primary cavityto form a preform; and (iii) drawing the second optical fibre from thepreform; wherein, the second optical fibre includes a core regioncomprising material that has been drawn from the first optical fibre, inwhich the preform includes a secondary elongate cavity that does notreceive an optical fibre.
 4. The method as claimed in claim 3, in whichthe primary cavity has a smallest transverse dimension of at least 100microns.
 5. The method as claimed in claim 3, in which a plurality offirst optical fibres are inserted into the primary cavity.
 6. The methodas claimed in claim 3, in which the preform element has a plurality ofprimary cavities into each of which a first optical fibre is inserted.7. The method as claimed in claim 3, in which the secondary cavity has across-sectional area smaller than that of any cavity in which an opticalfibre is received.
 8. The method as claimed in claim 3, in which thesecondary cavity is at most 500 microns in its largest transversedimension.
 9. The method as claimed in claim 3, in which a hole in thesecond fibre is formed from the secondary elongate cavity.
 10. Themethod as claimed in claim 9, in which the size of the hole, in thesecond fibre, formed from the secondary cavity is controlled bydifferential gas pressure.
 11. The method as claimed in claim 3, inwhich there are at least two secondary cavities.
 12. The method asclaimed in claim 11, in which the secondary cavities are arranged aroundthe primary cavities in the preform element transverse cross-section.13. The method as claimed in claim 11, in which the secondary cavitiesare arranged in a periodic pattern.
 14. The method as claimed in claim3, in which the preform element comprises a plurality of solid, elongatecylinders, which are drawn to form part of a cladding region of thesecond fibre.
 15. The method as claimed claim 3, in which the preformelement is formed by stacking a plurality of elements.
 16. The method asclaimed in claim 3, in which the first fibre is a standard fibre. 17.The method as claimed in claim 3, in which the first fibre is a photoniccrystal fibre.
 18. The method as claimed in claim 3, in which the secondfibre is drawn in a fibre-tapering rig.
 19. The method as claimed inclaim 3, in which a hole is prevented from forming in the second fibreby maintaining differential gas pressures in holes during drawing.
 20. Amethod of joining a first optical fibre to a second optical fibre, themethod comprising the steps of: (i) providing a preform elementcomprising material defining a primary elongate cavity; (ii) insertingthe first optical fibre into the primary cavity to form a preform; and(iii) drawing the second optical fibre from the preform; wherein, thesecond optical fibre includes a core region comprising material that hasbeen drawn from the first optical fibre, in which the preform element isformed by stacking a plurality of elements.
 21. The method as claimed inclaim 20, in which the primary cavity has a smallest transversedimension of at least 100 microns.
 22. The method as claimed in claim20, in which a plurality of first optical fibres are inserted into theprimary cavity.
 23. The method as claimed in claim 20, in which thepreform element has a plurality of primary cavities into each of which afirst optical fibre is inserted.
 24. The method as claimed in claim 20,in which the preform element comprises a plurality of solid, elongatecylinders, which are drawn to form part of a cladding region of thesecond fibre.
 25. The method as claimed in claim 20, in which at leastsome of the elements are glass tubes and/or rods.
 26. The method asclaimed in claim 20, in which at least one of the elements may itself beformed from a plurality of glass tubes and/or rods.
 27. The method asclaimed in claim 20, in which the elements are fused together.
 28. Themethod as claimed in claim 20, in which the stack is assembled on alarger scale and then drawn down in size to form the desired preformelement.
 29. The method as claimed in claim 20, in which the primaryelongate cavity is formed by omitting at least one element from thestack.
 30. The method as claimed in claim 20, in which the primarycavity is formed from the bore of a tubular element in the stack. 31.The method as claimed in claim 20, in which the first fibre is astandard fibre.
 32. The method as claimed in claim 20, in which thefirst fibre is a photonic crystal fibre.
 33. The method as claimed inclaim 20, in which the second fibre is drawn in a fibre-tapering rig.34. The method as claimed in claim 20, in which a hole is prevented fromforming in the second fibre by maintaining differential gas pressures inholes during drawing.
 35. A method of joining a first optical fibre to asecond optical fibre, the method comprising the steps of: (i) providinga preform element comprising material defining a primary elongatecavity; (ii) inserting the first optical fibre into the primary cavityto form a preform; and (iii) drawing the second optical fibre from thepreform; wherein, the second optical fibre includes a core regioncomprising material that has been drawn from the first optical fibre, inwhich the first fibre is a photonic crystal fibre.
 36. The method asclaimed in claim 35, in which the primary cavity has a smallesttransverse dimension of at least 100 microns.
 37. The method as claimedin claim 35, in which a plurality of first optical fibres are insertedinto the primary cavity.
 38. The method as claimed in claim 35, in whichthe preform element has a plurality of primary cavities into each ofwhich a first optical fibre is inserted.
 39. The method as claimed inclaim 35, in which the preform element comprises a plurality of solid,elongate cylinders, which are drawn to form part of a cladding region ofthe second fibre.
 40. The method as claimed in claim 35, in which thefirst fibre is a standard fibre.
 41. The method as claimed in claim 35,in which the second fibre is drawn in a fibre-tapering rig.
 42. Themethod as claimed in claim 35, in which a hole is prevented from formingin the second fibre by maintaining differential gas pressures in holesduring drawing.
 43. An optical device comprising a first optical fibreand a second optical fibre having a core region and a cladding region,wherein the core region of the second optical fibre comprises materialthat has been drawn from the first optical fibre, in which the secondfibre is a photonic-crystal fibre.
 44. An optical device as claimed inclaim 43, in which there is a gradual transition from the first fibre tothe second fibre.
 45. An optical device as claimed in claim 44, in whichthe transition shape is gradual enough to satisfy the criterion foradiabaticity, such that the transformation device carries light betweenthe first fibre and the second fibre with less than 3 dB of loss.
 46. Anoptical device as claimed in claim 43, in which the device includes alength of preform element between the first optical fibre and the secondoptical fibre.
 47. An optical device as claimed in claim 43, in whichthe second fibre includes at least one elongate hole.
 48. An opticaldevice as claimed in claim 43, in which the second fibre is a standardfibre.
 49. An optical device as claimed in claim 43, in which the secondcore incorporates dopants.
 50. An optical device as claimed in claim 43,in which the second fibre includes a jacket region.
 51. An opticaldevice as claimed in claim 43, in which the first fibre is a standardfibre.
 52. An optical device as claimed in claim 43, in which the firstfibre is a photonic crystal fibre.
 53. An optical device as claimed inclaim 43, comprising a third optical fibre.
 54. An apical device asclaimed in claim 53, in which the core region of the second opticalfibre includes material drawn from the third optical fibre.
 55. Anoptical device claimed in claim 53, in which the second optical fibreincludes a second core region containing material drawn from the thirdoptical fibre.
 56. An optical device comprising a first optical fibreand a second optical fibre having a core region and a cladding region,wherein the core region of the second optical fibre comprises materialthat has been drawn from the first optical fibre, in which the firstfibre is a photonic crystal fibre.
 57. An optical device as claimed inclaim 56, in which there is a gradual transition from the first fibre tothe second fibre.
 58. An optical device as claimed in claim 57, in whichthe transition shape is gradual enough to satisfy the criterion foradiabaticity, such that the transformation device carries light betweenthe first fibre and the second fibre with less than 3 dB of loss.
 59. Anoptical device as claimed in claim 56, in which the device includes alength of preform element between the first optical fibre and the secondoptical fibre.
 60. An optical device as claimed in claim 56, in whichthe second fibre includes at least one elongate hole.
 61. An opticaldevice as claimed in claim 56, in which the second fibre is a standardfibre.
 62. An optical device as claimed in claim 56, in which the secondcore incorporates dopants.
 63. An optical device as claimed in claim 56,in which the second fibre includes a jacket region.
 64. An opticaldevice as claimed in claim 56, in which the first fibre is a standardfibre.
 65. An optical device as claimed in claim 56, comprising a thirdoptical fibre.
 66. An optical device as claimed in claim 65, in whichthe core region of the second optical fibre includes material drawn fromthe third optical fibre.
 67. An optical device as claimed in claim 65,in which the second optical fibre includes a second core regioncontaining material drawn from the third optical fibre.